This invention relates to an automatic transmission lockup clutch control apparatus for use with an automotive vehicle.
Automatic transmission lockup clutch control apparatus have been proposed, for example, in Japanese Patent Kokai No. 2-236057, to operate a lockup clutch provided in an automatic transmission in a slip lockup mode so as to bring a difference between the engine speed and the turbine speed to a predetermined value. The conventional lockup clutch control apparatus calculates the turbine speed based upon the transmission output shaft speed and the gear ratio calculated for the selected gear shift mode.
The conventional lockup control apparatus requires no turbine speed sensor and has the advantages in easy application to existing automatic transmission and less space consumption. However, the conventional lockup control apparatus cannot provide a constant degree of slip in the lockup clutch over a long period of time since the gear ratio estimated for a specified one of various gear change modes will change because of changes of the lockup clutch with time, engine output torque fluctuations and variations in the friction factor of the friction surface of the lockup clutch.